The invention relates to a leveling instrument having an imaging optical system and a spatially resolving opto-electronic detector for image recording, and having an electronic system and an arithmetic unit for controlling the detector and for image evaluation.
Devices of the said type are disclosed in German Publication DE 34 24 806 C2 and U.S. Pat. No.4,715,714. The leveling instrument described there determines the distance and the height difference relative to leveling rods set up at a distance. It therefore serves in geodetic surveying for determining benchmarks and for topographic and cartographic surveying. However, it is also used in construction surveying, in building traffic ways or in building tunnels and in mining.
In the classic leveling instrument, its telescope is used by the operator for the purpose of visually reading off the numerical value on a height scale of the leveling rod. The numerical value read off is located exactly on the optical axis in the cross hairs of the telescope.
The advent of automated digital leveling instruments rendered it possible for the first time to read off rods electronically. For this purpose, a leveling rod has been developed on which a code pattern composed of black and white elements is applied. In accordance with DE 34 24 806 C2, the portion of the code pattern situated in the field of view of the telescope optical system of the digital leveling instrument is imaged on a spatially resolving optoelectronic detector group. In this process, use is made not only of the code pattern information located on the optical axis of the telescope, but also of the code pattern information located in the entire field of view of the telescope, in order to determine the height value. In addition, the position sensor of the focusing device of the digital leveling instrument supplies the approximate distance from the leveling rod. The exact distance is determined by the evaluation procedure of the recorded code pattern.
The leveling operation runs as follows. The leveling rod is sighted using the telescope of the leveled leveling instrument, and focused using the focusing device. The position sensor of the focusing device supplies the distance from the leveling rod, from which, together with the focal length of the telescope objective, the distance scale is calculated. This distance scale is included in the code pattern, in order to be able to carry out a comparison with a reference code pattern. The latter is located as an original code pattern of the leveling rod in an electronic memory. A cross correlation which determines a best possible agreement of the measured code pattern section with a corresponding section on the reference code pattern is carried out as comparison method. The agreement found reveals the sighted location on the leveling rod, and thus the height of the leveling rod with respect to the leveling instrument.
Although the actual leveling and measuring operation is performed fully automatically, it is necessary for the imaging optical system of the leveling instrument to be set in advance such that the leveling rod is sharply imaged in the intermediate image plane of the telescope objective. For this purpose, a focusing lens can be mechanically displaced along the optical axis of the telescope optical system. The focusing lens is firmly connected to a focusing drive. The focusing drive is actuated manually and at the same time the image of the leveling rod is observed through the telescope eyepiece. The focusing operation is terminated when the image of the leveling rod appears sharp.
An electronic leveling instrument of the said type is also described in EP 0 576 004 A1. A telescope is used as imaging optical system. The telescope is focused onto a leveling rod whose distance and height are to be determined with respect to the leveling instrument. A black and white pattern of lines is applied to the leveling rod. The objective of the telescope picks up that part of the pattern of lines of the leveling rod which is located in its field of view, and generates therefrom an image on a photoelectric receiver. The electric signals of the receiver are evaluated in a signal processor directly or via a Fourier transformation. In the process, the distance between the leveling instrument and leveling rod, and the leveling height are determined from the period and from the phase of the pattern of lines picked up by the telescope objective and evaluated by the photoelectric device.
The focusing of the telescope onto the leveling rod is performed manually. For this purpose, a focusing lens is displaced along the optical axis of the telescope until the image of the leveling rod observed through the telescope eyepiece appears sharp.
in another design, the focusing lens is motorized, and this renders automated focusing possible. In this case, the focusing lens is moved in steps with the aid of a motor control. With each step, the detector records the pattern of lines of the leveling rod, and the signal processor forms the Fourier transform therefrom. The height of the peak, corresponding to the periodic structure of the pattern of lines, of the Fourier transform is a measure of the focusing. The maximum peak height of this Fourier transform signifies optimum focusing. Consequently, the focusing lens is brought to this position after passing through the maximum peak height.
The two digital leveling systems according to DE 34 24 806 C2 and U.S. Pat. No. 4,715,714 and to EP 0 576 004 A1 require moving optical and mechanical components to focus the leveling rod. Such components, which are moved mechanically in relation to one another, have to be produced and adjusted with care. This complicates production and increases cost.
In addition, the manual focusing is somewhat troublesome for the user of the leveling instrument: the user has to look through the eyepiece of the telescope and subjectively estimate the focusing of the leveling rod image. Although the motorized version of the focusing lens relieves the user of this work, this is achieved at the expense of a high outlay for measurement. Thus, the leveling rod image must be recorded by the detector and processed in a microprocessor with the aid of suitable algorithms. The computational result must be converted by an electronic motor control, the electric motor must be driven, and the focusing lens must be moved to the appropriate position. The position must be known via a position sensor or, in the case of the use of a stepping motor, at least via the counting steps, in order to be able to move back to this position again after passing through the optimum focusing position. Moreover, the probability of the failure of an instrument is increased by the additional extent of software, electronics and, in particular, electromechanics, because of the finite service life of the electric motor.